1. Field of the Invention
This invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device within a semiconductor substrate having uneven surface portions.
2. Description of the Prior Art
In a semiconductor integrated circuit (hereinbelow, termed "IC"), a large number of circuit elements such as transistors, diodes and resistances are formed within a common semiconductor substrate. The respective circuit elements must be electrically isolated by isolation regions. The formation of the isolation regions is generally carried out by a diffusion process.
In order to enchance the density of integration in the IC, the number of accommodated circuit elements per unit area must be made large. To this end, it is necessary to reduce the area of the isolation region occupying a considerable space in the circuit element. However, where the isolation region is formed by diffusion as stated above, it is difficult to reduce the area thereof to below a certain fixed value. It has, therefore, been desired to reduce the occupying area by any method.
In order to eliminate the disadvantage described above, there has been proposed a method wherein the isolation is effected with an insulator which replaces the diffused region. According to this method, a region within a semiconductor substrate prearranged for isolation is formed with a semiconductor oxide film by applying a local oxidation technique. The oxide film is utilized as the insulator, and is capable of reducing the occupying area by 40 - 50 % in comparison with a diffused region. The local oxidation technique is described in detail in, for example, "Philips Research Report," pages 118 through 132 issued by Philips Reseach Laboratory in April 1970.
FIGS. 1(A) and 1(B) illustrate an example in the case of manufacturing an IC by applying a local oxidation technique as explained above, and show only one circuit element. First, as shown in FIG. 1(A), an N conductivity type silicon substrate 1 is prepared, and a film 2 of, for example, silicon nitride (Si.sub.3 N.sub.4 ) is formed on a desired area of the surface of the substrate 1 as an oxidation resisting film. As a method of forming the Si.sub.3 N.sub.4 film 2, there can be applied a known method which exploits the vapor phase reaction between, for example, monosilance (SiH.sub.4 ) and ammonia (NH.sub.3 ). It is preferred that a thin SiO.sub.2 layer is formed on the surface of the substrate prior to the vapor phase reaction so as to prevent the substrate surface from becoming rough. The film 2 being about 1,300 A thick can be formed only at the desired area in such a way that, after forming the Si.sub.3 N.sub.4 film over the entire area of the surface of the substrate 1, it is subjected to the selective etching within an etchant such as heated phosphoric acid (H.sub.3 PO.sub.4 ). Thereafter, using the film 2 as a mask, the substrate is somewhat etched. The amount of etching is to such an extent that SiO.sub.2 is prevented from protruding beyond the substrate surface on account of a volumetric increase due to oxidation processing to be subsequently carried out.
As illustrated in FIG. 1(B), the resultant substrate 1 is heat-treated during exposure to an oxidizing atmosphere, to locally oxidize that area of the substrate surface on which the film 2 does not exist. By way of example, the heat treatment is carried out at 1,000.degree. C. for 16 hours, whereby a silicon oxide (SiO.sub.2 ) film 3 having a thickness of about 18,000 A can be produced. Subsequently, the film 2 is removed, and the exposed part of the substrate 1 is doped with a necessary impurity by an expedient such as diffusion and ion implantation. Thus, the desired circuit element is formed. The figure exemplifies a case of forming a transistor, in which numeral 4 designates an emitter region, numeral 5 a base region and numeral 6 a collector region. Shown at 7 is a protective (SiO.sub.2 ) film which is formed on the surface of the substrate.
Numeral 8 indicates an emitter electrode and numeral 9 a base electrode, and these electrodes are formed by photoetching the protective film 7 and thereafter depositing aluminum or the like. The silicon oxide film 3 functions as the isolation region for insulation among the circuit elements.
When an IC, applying the local oxidation technique as stated above, is observed, it is noted that a conspicuous protuberance is formed especially at the upper end part of the silicon oxide film 3 produced by the local oxidation. The protuberance is usually called a "bird head"or "bird beak." The existence of the protuberance incurs many problems such that the electrode wirings 8 and 9 formed thereon are prone to be disconnected and that the close contact with a mask pattern for a photolithographic processing is rendered imperfect.